Rapid urbanization occurs when populations move into cities at a rate that is faster than the development of infrastructure. This is usually the result of economic changes that leave rural dwellers and farmers in poverty. This migration to cities has been going on in developing countries for decades, resulting in enormous shanty towns surrounding growing cities such as Rio, Mexico City and Shanghai.
Human activity that isn't controlled through appropriate infrastructure leads to damage to the natural environment, and this damage increases with larger populations. Inadequate sewer facilities lead to polluted water, unregulated growth leads to housing being built in environmentally sensitive areas and a lack of gas or electricity leads to intensive cooking with wood fires, something that seriously compromises air quality. Population increase, which is higher in countries with lower education levels and less empowered women, make all of these problems worse.
Lack of Infrastructure
In a well-planned city, infrastructure is created in a sensible way to accommodate the population. Electric grids, sewer facilities, gas lines and roads are expanded at the same rate as the population. When rapid urbanization occurs, many of the new residents of a city are there unofficially, living in informal slums and shanty towns that have inadequate or nonexistent public services. With no census or official information, it is difficult or impossible for municipal authorities to plan for or provide adequate infrastructure for burgeoning populations.
Poor farmers and landless people gravitate to cities in search of work and an improved standard of living, but often find themselves living in poverty in the city and unable to find work. Since employment levels are related to levels of economic activity and development of infrastructure, it isn't surprising that cities with many shanty towns have high levels of unemployment. The economic base of such a city is dwarfed by the numbers of people. With such a surplus of willing workers, this situation drives wages down, meaning that even when a poor person has a job, it may pay very little.
Diseases that are caused by unclean water, impure air and overcrowding are epidemic in some rapidly growing cities. Cholera, diarrhea and other water-borne illnesses thrive in places that don't have water filtration plants, while lung diseases and breathing problems are caused by dirty air. Rapidly expanding cities are often characterized by huge traffic problems, something that contributes to unclean air. All of these health problems are made worse when people don't have access to reliable and low-cost health care.